Hi,
What do you want to measure? Is it the amplitude of the sine wave or is is the frequency? Or both?
What frequency has the input signal? If it is slow enough, the ADC can measure it.
Does the AC signal have a DC component, or is the AC signal symmetric round ground.
If it doesn't, a high-pass AC filter must be installed. The high-pass frequency must be low enough not to affect the AC signal.
An alternative is to use an active "diod" which will remove the negative pulses.
If the AC signal is too high for direct measurement, an active low pass- or peak filter must be used.
The low pass/peak filter could have a bleeder resistor, so that if the signal drops, the filter output will drop.
The other way is to include a "reset" input to the filter, so after each measurement, the filter is reset, and can then measure the maximum input voltage.
If a reset function is used, the filter must run for at least 1/f seconds before the ADC is started.
Regards,
Ake